For a typical MIMO system supporting precoding and rank adaptation, a transmitter, comprised in the system, need information about the current channel rank (rank indicator-RI), preferred precoder matrix indicator (PMI) and channel quality indicator (CQI) from terminals (or mobile terminals), comprised in the system. This information can either be jointly transmitted in a feedback channel, such as PUCCH (Physical Uplink Control Channel), or the information can be separately transmitted either in different time intervals or even on different physical channels.
There can be reasons for separating, at least rank information from CQI/PMI information. One reason is that it is believed that the channel rank varies more slowly than the channel quality. Since the PMI is strongly connected to the CQI, for a certain channel realization, different precoders will result in different CQI. Another reason can be that the pure size of the control data (RI+CQI+PMI) will limit the range of the control channel, i.e. too many physical bits should be transmitted in a certain time frame.
Hence, it is a problem that the rank information is sent more often than necessary, and that the control data includes more bits than the number of bits in a certain time frame, resulting in an inefficient use of the control channel.
It should also be noted that the format of the CQI/PMI report will depend on the rank.
The document OUALCOMM EUROPE: “Details on CQI Format” 3GPP DRAFT; R1-074971, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE; 650, ROUTE DES LUCIOLES; F-06921 SOPHIA-ANTIPOLIS CEDEX; FRANCE, vol. RAN WG1, no. Korea; 20071105, 29 Oct. 2007 (2007-10-29), discusses uplink control channel payload, based on the codebook size.
The document INTERDIGITAL COMMUNICATIONS ET AL: “E-UTRA PUCCH: ACK/NACK, CQI, PMI, and RI Issues” 3GPP DRAFT; R1-074391, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE; 650, ROUTE DES LUCIOLES; F-06921 SOPHIA-ANTIPOLIS CEDEX; FRANCE, vol. RAN WG1, no. Shanghai, China; 20071008, 3 Oct. 2007 (2007-10-03), provides a summary discussion concerning the reporting of CQI, PMI, ACK/NACK and RI on PUCCH.
The document, HUAWEI: “cal and PMI resource management” 3GPP DRAFT; R1-074233, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE; 650, ROUTE DES LUCIOLES; F-06921 SOPHIA-ANTIPOLIS CEDEX; FRANCE, vol. RAN WG1, no. Shanghai, China; 20071008, 2 Oct. 2007(2007-10-02), discusses the feedback of channel state information from the UE to the NodeB.
Further, the document ERICSSON ET AL: “Corrections to CQI and RI fields description”3GPP DRAFT; R1-090533 CORRECTIONS TO CQI AND RI FIELDS DESCRIPTION B, 3RD GENERATION PARTNERSHIP PROJECT (3GPP), MOBILE COMPETENCE CENTRE; 650, ROUTE DES LUCIOLES; F-06921 SOPHIA-ANTIPOLIS CEDEX ; FRANCE, no. Ljubljana; 20090112, 12 Jan. 2008 (2008-01-12) is related to corrections of CQI and RI fields description.